High-intensity carbon electrode



Patented May 23, 1950 2,508,455 HIGH-INTENSITY CARBON ELECTRODE ClarenceE. Greider, Lakewood, Ohio, assignor, by

mesne assignments, to Union Carbide and Carhon Corporation, acorporation of New York NoDrawing. Application July 6, 1949, Serial No.103,327

Claims.

1. 1 This invention relates to an improved positive, high intensity,carbon electrode for use in motion picture projectors and the like, bymeans of which improved arc stability is achieved through theintroduction of a small amount of tungsten to said electrode.

Carbon electrodes are of two general classes, referred to in the art aslow intensity and high intensity carbons. Low intensity positive carbonspresently in use vary in diameter from about 9 to 14 mm. and theoperating amperage is in the nature of 10 to 52 amperes for theserespective electrode size variations The low intensity carbon arc isseldom operated at a current density much over 200 amperes per squareinch cross-section of the positive electrode.

In general, a high intensity carbon arc is distinguishable from a lowintensity carbon arc in several important respects. The former has acore containing a substantial amount of flame material which becomeshighly luminescent under the action of the electron bombardment in thearc stream. The current density in high intensity positive carbons isincreased to values which may exceed 1500 amperes per square inch. Atthis high current density the material of the core is vaporized morerapidly than that of the carbon shell until a deep, cup-like crater isformed in the carbon face. Within this crater vapors of carbon and corematerials are excited to a very high temperature and radiatingefiiciency, producing a brightness several times that possible at thepositive crater of the low intensity carbon arc.

In operation, high intensity carbon arcs are of two types. In the oldertype, the negative carbon is inclined to the positive and the latter iscontinuously rotated to maintain a symmetrical crater form. This type ofhigh intensity are generally employs a positive carbon varying from 9 to16 mm. in diameter, with are amperages varying from 60 to 1'70 amperes.

A newer type of high intensity are is operated without rotation of thepositive carbon and with the positive and negative carbons in coaxialalignment. In this arc the positive carbon (usually referred to as asimplified high intensity type carbon) has a size range from 6 to 8 mm.in diameter and operates at amperages varying from to 70 amperes.

The operation of a high intensity carbon are at the lowest currentdensity consistent with the light requirements of a particular motionpicture projector installation results in economies with regard to bothcarbon cost and operating power consumption. With amperages on theborder line of that required for a high intensity eiiect, however,frequent diihculty arises due to the tendency of the arc to go in andout of high intensity, i. e. to fluctuate between a high intensity andlow intensity are. As the light brilliancy of the high intensity are isseveral times that of the low intensity are the resultis a disturbinglight fluctuation on the motion picture screen; The exact currentdensity at which this tendency to go in and out of high intensity takesplace varies with the different carbons depend' ing on their size,composition and designed operating characteristics. Generally it is inthe range of 450 to 600 amperes per square inch. While this difficultymight be overcome by changing the carbon size or current load, this isusually not convenient or feasible, because of the standardizedprojection equipment generally in use in the trade.

Accordingly, the object of this invention is to provide an improved highintensity positive carbon, which is operable over a wider currentdensity range with a stable high intensity arc,

without the necessity of changing the carbon size or operatingcharacteristics of a projection system wherein established carbon sizesare to be used.

I have discovered that the addition of a small amount of tungsten,preferably as tungstic oxide, to the positive carbon greatly reduces thecurrent density at which a carbon arc fluctuates between the high andlow intensity effect. While the tungsten may be added to the shell, coreor both, it will generally be incorporated in the core mix as thesimplest and most convenient way of introducing it into the carbon. Thesame desirable effects are produced, however, if the tungsten is presentin the shell or in both the shell and core.

The most advantageous amount of tungstic oxide will vary somewhat withdiiierent carbon electrode compositions. In general, between 0.1 and 0.5percent by weight of the core or shell will provide satisfactoryresults, i. e. 0.1 to 0.5 percent by weight of the core composition whenthe tungstic oxide is added to the core or 0.1 to 0.5 percent by weightof the shell composition when the tungstic oxide is added to the shell.Amounts in excess of 0.5 percent increase the burning rate somewhat thusreducing carbon efficiency slightly (i. e. lumen-hours per inch). Whileunder certain circumstances it might be desirable to increase thetungstic oxide to an amount in excess of 0.5 percent, in spite of theslight decrease in carbon efilciency (lumen hours per inch), in no caseshould it exceed 1 percent, as an amount in excess of this will producecore fusion of the carbon. If less than 0.1 percent tungstic oxide isemployed in the core mixture, the stabilizing eifect becomes somewhaterratic.

While tungstic oxide in amounts between 0.5 and 1 percent may reduce thecarbon efliciency somewhat as expressed in lumen hours per inch, thecarbon efficiency as expressed in lumens per ampere is increased by theaddition of tungstic oxide throughout the useful range of 0.1 to 1percent. In addition, because the inclusion of tungstic oxide in thecore material produces a higher are color temperature (the light beingsomewhat bluer) the apparent brilliancy of the light produced is greaterthan the actual lumen measurement indicates. I

The invention is applicable to the improvement of all types of highintensity positive carbons,

which may vary considerably in the exact shell and core composition. Theuseful results are especially significant in the simplified highintensity carbons employed in the reflector type projection systems.These carbons generally have a shell composition containing a highproportion of coke-base carbon, with a core mix having predominatingamounts of rare earth fluorides and oxides. In such carbons tungsticoxide in the core, in an amount of about 25% by, weight thereof,provides a substantial improvement in the ability of the carbon tomaintain a'stable high intensity are at an amperage lower than thatnormally required to insure a constant high intensity effect in the arc.

It will be readily apparent to those skilled in the arc that while Ihave described my invention with particular reference to improved arcstability in high intensity, positive carbons of present' commercialimportance, it will be equally useful to the trade in the design of moreefficient carbons capable of operating at lower current density (i. e.smaller carbon diameter or operating ampera e or both) with good arestability and a high intensity arc.

. I claim:

1. In a positive, high intensity, carbon electrode, the improvementwhich is a shell and a core one of which contains tungsten in an amountbetween 0.1 and 1 percent by weight computed as tungstic oxide.

2. In a positive, high intensity, cored, carbon electrode, theimprovement which is a core con- 4 taining tungsten in an amount between0.1 and 1 percent by weight computed as tungstic oxide.

3. In a positive, high intensity, cored, carbon electrode, theimprovement which is a core containing tungstic oxide in an amountbetween 0.1 and 1 percent by weight.

4. An improved positive, high'intensity, carbon electrode comprising ashell and a core one of which contains between 0.1 and 1 percent byweight of tungsten computed as tungstic oxide.

5. An improved positive, high intensity, carbon electrode comprising ashell and a core, said core containing between 0.1 and 1 percent byweight of tungsten computed as tungstic oxide.

6. An improved positive, high intensity, carbon electrode comprising ashell and a core, said core comprising a mixture of carbon, rare earthoxides and fluorides and 0.1 to 1 percent by weight of tungstic oxide. 7

7. An improved positive, high intensity, carbon electrode comprising ashell and a core, saidcore containing between 0.1 and 0.5 percent byweight of tungstic oxide.

8. An improved positive, high intensity, carbon electrode comprising ashell composed in major proportions of coke-base carbon, and a coreconsisting predominately of rare earth oxides and fluorides, said corefurther containing about 0.25% by weight of tungstic oxide.

9. An improved positive, high intensity, carbon electrode comprising ashell and a core, said electrode being within 6 to 8 mm. in diameter,said shell being composed in major proportion of coke-base carbon andsaid core consisting predominately of rare earth oxides and fluorides,said core further containing about 0.25% by weight of tungstic oxide.

10. In the operation of a simplified high intensity carbon electrode arehaving a non-rotat ing positive electrode and a coaxially alinednegative electrode, the improvement which comprises continuallysupplying tungsten to the positive electrode crater from a positiveelectrode core composition containing 0.1 to 1 percent by weight oftungsten computed as tungstic oxide.

' CLARENCE E. GREIDER.

REFERENCES orrED UNITED STATES PATENTS Name Date Hardman et al Sept. 2,1930 Number

1. IN A POSITIVE, HIGH INTENSITY, CARBON ELECTRODE, THE IMPROVEMENTWHICH IS A SHELL AND A CORE ONE OF WHICH CONTAINS TUNGSTEN IN AN AMOUNTBETWEEN 0.1 AND 1 PERCENT BY WEIGHT COMPUTED AS TUNGSTIC OXIDE.